Use IPPROTO_TCP instead of SOL_TCP for cross-platform compatibility.
xbuild: use the AdditionalReferencePath items to locate assemblies

This is a little underdocumented feature of MSBuild. Items with the name
"AdditionalReferencePath" get searched for referenced assemblies. This helps
with some scenarios, e.g. when the assembly has to be references from a central
location.
Allow clang to compile sgen

Clang has supported both the `__thread` and `tls_model` attributes (needed to compile the sgen GC) since Clang/LLVM 3.2.

When compiling Mono with clang, the check which sets `with_tls=pthread` causes `mono-sgen` to be compiled anyway, although with some improper configuration which causes it to crash immediately when run.

I also removed the check for clang which was causing the `HAVE_TLS_MODEL_ATTR` symbol not to be defined, as Clang has in fact supported `tls_model` for some time now.

I cloned the Mono repository today, applied these changes, and compiled Mono using the LLVM/Clang 3.4 nightly builds on both the 32- and 64-bit versions of Ubuntu 12.04 LTS. In both cases, Mono compiled without any problems at all. Running `make check` on both boxes yielded no errors, except that the `finalizer-exception.exe` test hung on the x64 build; it's unclear whether or not this is a bug which manifests itself when Mono is built with clang, or if it's just a general bug in Mono. Either way, it doesn't seem to affect anything else.

If there are any issues compiling Mono with clang on other platforms, these "global" checks for clang should still be removed, and new platform-specific checks added where necessary.
Enable sigaltstack and dtrace on FreeBSD

Enable the use of sigaltstack and DTrace on FreeBSD. Note that while DTrace is enabled here (with `has_dtrace=yes`), the DTrace hooks will only be compiled into Mono when the FreeBSD kernel has been compiled with DTrace support.
Do system.object type scan after all references are loaded. Fixes #12991
Merge pull request #679 from jack-pappas/patch-6

Enable sigaltstack and dtrace on FreeBSD
Merge pull request #572 from jack-pappas/sockets-ipproto

Use IPPROTO_TCP instead of SOL_TCP for cross-platform compatibility.
Merge pull request #678 from jack-pappas/patch-5

Allow clang to compile sgen
Merge pull request #649 from DavidS/feature/implement-additional-reference-path

xbuild: use the AdditionalReferencePath items to locate assemblies